The Best Guess at What 'Planet Nine' Looks Like

The astronomical community is abuzz with the possibility that a ninth planet exists in the far reaches of the solar system. A new study by European scientists imagines what this hypothetical planet might look like, revealing important insights as to how we might actually find it.

Astrophysicist Christoph Mordasini from the University of Bern and his PhD Student Esther Linder are planet modelling specialists, and they recently applied their expertise to the figuring out what Planet Nine might look like. Their ensuing analysis, which has been accepted by the science journal Astronomy & Astrophysics, paints a fascinating portrait of a dark and cold planet in the far reaches of the solar system.

The purpose of the exercise was to create ballpark estimates for the planet’s radius, temperature, brightness, and most importantly level of thermal radiation. The last item is of particular interest because while Planet Nine may be too dim to be seen with our current telescopes, it’s thermal signature might be detectable by other means. Encouragingly, the simulations created by Mordasini and Linder suggest this may very well be the case.

With very little data to go on, the researchers decided to simulate several different scenarios. For the study, the astrophysicists assumed that Planet Nine is a smaller version of Uranus and Neptune. They modelled hypothetical planets that were five, 10, 20 and 50 times heavier than Earth, and at distances of 280, 700, and 1,120 AU from the Sun (1 AU being the average distance of the Earth to the Sun; for comparison, Pluto is about 40 AU from the Sun). One particular simulation jumped out at the researchers as plausible.

“For me candidate Planet Nine is a close object, although it is about 700 times further away as the distance between the Earth and the Sun,” noted Linder in a statement. The “ideal” Planet Nine, according to the models, features a mass ten times heavier than Earth, and a radius 3.7 times wider than our planet. Similar to Uranus and Neptune, it has an outer envelope of helium and hydrogen, a layer of gas (also consisting of helium and hydrogen), a water ice layer, a silicate mantle, and an iron core.

The models also projected a temperature of 47 Kelvin (-374 degrees Fahrenheit, -226 degrees Celsius). Planet Nine is bitterly cold—but this data suggests that it’s being heated from the inside.

“This means that the planet’s emission is dominated by the cooling of its core, otherwise the temperature would only be 10 Kelvin,” explained Linder.“Its intrinsic power is about 1,000 times bigger than its absorbed power.”

So, Planet Nine’s reflected sunlight contributes a very tiny part of the total radiation that could be detected on Earth (it’s exceptionally dim, less than 1 percent as bright as Jupiter). But it also means that this nominal planet is much brighter in the infrared than in the visual. As the researchers put it, Planet Nine is a “self-luminous planet.”

That’s good news for astronomers, who can now scan the heavens for these thermal signatures. All this is quite remarkable; even though we’ve never actually seen this thing, it’s actually starting to take shape.